This invention relates to an integrated process for the anodization of aluminum electrolytic capacitor foil. A hydrous layer is first formed on the foil, and then it is electrochemically anodized in a bath containing boric acid and 2 to 50 ppm phosphate at a pH of 4.0 to 6.0. Anodization is interrupted to stabilize the foil by passing it through a bath containing a mildly alkaline borax solution at a temperature above 80.degree. C. Thereafter, the foil is reanodized in the boric acid electrolyte. Foil sutiable for use in electrolytic capacitors for up to 760 V service is produced by this process.
Improvements have been made both in the manufacture of aluminum foil for electrolytic capacitors and in the etching of such foil resulting in the capability of producing higher voltage foil than had been possible until recently. The improvements resulted in a need for anodization processes capable of producing higher voltage dielectric oxide films to take advantage of these newer foils and etching processes.
It has been customary to form a hydrous oxide layer on aluminum foil prior to anodization of the foil for service above about 200 V. Usually this hydrous oxide layer is formed by passing the foil into boiling deionized water. This layer permits anodization to above 200 V and permits power savings during anodization and a higher capacitance per given anodization voltages. Although the use of a hydrous oxide layer is not new, the mechanism by which it produces the above results is still not understood.
The prior art has shown the use of borate and citrate electrolyte for anodization up to 500 V, generally up to about 450 V. The anodization process which was capable of producing 500 V foil was an excessively lengthy and cumbersome process not suitable for present day manufacturing schemes. In particular, the stabilization or depolarization time required was excessively long.
This stabilization or depolarization is needed as it is well-documented that aluminum capacitor foil after apparently complete formation of a high voltage dielectric oxide film evidences instability as shown by a sudden loss of field strength. This behavior is most markedly observed when the foil also bears a hydrous oxide layer formed prior to anodization. There is general agreement in the electrolytic capacitor industry that this dielectric instability is caused by the creation of voids within the formed dielectric oxide layer. It has been further postulated that oxygen gas is trapped within these voids and is liberated during the stabilization or "depolarization" treatment that bring about a relaxation in the strength of the dielectric.
Whatever the actual physical mechanism which may be involved, it is known in the prior art to remedy the situation by various so-called depolarizing techniques--heating, immersion in hot water with and without various additives, mechanical flexing, pulsed currents, current reversal, or a combination of these--in short, methods which tend to relax or crack the dielectric barrier layer oxide so that these voids may be filled with additional dielectric oxide and thereby impart permanent stability to the oxide film.
One such process is described by Walter J. Bernard in a copending application filed on even date herewith. His process involves passing anodized foil through a bath containing preferably an aqueous borax solution having a pH of 8.5 to 9.5 at a temperature above 80.degree. C. While boric acid or borax at acidic pH controls the hydration of aluminum foil, at the mildly alkaline pH above, borax is more effective than the hot water reaction in opening up the dielectric film. In addition to opening up this film, it seems to attack the excess hydrous oxide present without damaging the barrier layer dielectric oxide and leads to the formation of a stable dielectric oxide upon subsequent reanodization of the foil.